High-average power (HAP) solid-state lasers (SSL) are finding increasingly important utility in defense and commercial applications. Because a laser gain medium must be excited by an outside source of energy before it will emit electromagnetic radiation, SSLs generally comprise a laser gain medium and a source of optical energy. This source of energy provides visible or near visible optical pump radiation into the laser gain medium, which converts it into a laser beam. In this regard, most of the recent growth in the SSL industry can be attributed to introduction of laser diodes as the source of energy because diodes are generally very efficient in converting electric energy into the pump light and usually deposit only a limited amount of waste heat into the laser gain medium. Generally, diode-pumped SSLs are advantageous over the more conventional gas lasers because diode-pumped SSLs operate at wavelengths compatible with optical fibers, have complete electric operation, a short wavelength, a continuous duty and high efficiency. Additionally, diode-pumped SSLs allow engineering a high-power device into a small, lightweight package.
Many emerging military and industrial applications for HAP SSL require integration of laser systems on mobile platforms, such as vehicles, watercraft, spacecraft and aircraft. For example, military applications for mobile HAP SSLs can include defense against tactical and strategic missiles. Also, mobile HAP SSLs can address emerging industrial applications, such as dismantlement of nuclear facilities, drilling of oil wells and road tunnels, and spacecraft orbit transfer.
As previously indicated, SSLs operate on electricity. Despite the recent introduction of pumping SSLs by highly efficient semiconductor diodes, the operation of SSLs still produces a significant amount of waste heat that must be rejected. Typically, for each joule of laser energy produced, three to four joules of heat must be removed from a SSL system and rejected to the environment. In conventional SSL systems, the SSL pump diodes and laser gain medium are operated at ambient and sub-ambient temperatures, which increases the electric efficiency of the SSL system and extends the life of the pump diodes. To reject heat to the environment at the same or higher temperature typically requires the use of refrigeration to “pump” the heat from laser components to the environment at a higher temperature. In this regard, FIG. 1 illustrates a simplified diagram of a closed loop refrigeration system 5 commonly used in SSL systems. A variety of cooling systems of this type are commercially available and currently used in many applications. However, closed cycle refrigeration type systems are usually very bulky and heavy in comparison to the SSL they support. While this is not of particular concern in fixed laser installations (e.g., factory use), a closed-cycle refrigeration is entirely unsuitable for large (multi-kilowatt) SSLs operated on mobile platforms where size and weight are paramount. In addition, closed cycle refrigeration systems are expensive and require large amount of electric power to operate (typically 20–30% of the SSL system electric power budget).
Referring to FIG. 2, another conventional refrigerator system 7 used with SSL uses open-cycle with subcritical refrigerant storage. Such refrigerants must be sufficiently safe for release into the environment, hence the choice is limited to liquid carbon dioxide and liquid nitrogen. These cryorefrigerants are stored in dewars (typically cooled by the gradual boil-off of stored cryogen) and are normally used in conjunction with a heat exchanger and secondary coolant loop. The resulting system is complex, heavy, and requires frequent user's attention. Furthermore, expendable cryorefrigerant must be periodically replenished, which requires significant logistics support in the field. Requirement for such logistics support may be a barrier to many important military and commercial uses.